What is an ISO week number?

ISO 8601 defines weeks as Monday-to-Sunday. Week 1 is the week containing the year's first Thursday (equivalently, 4 January). Week numbers run 01–53.

Why might the week-year differ from the calendar year?

For dates near 1 January or 31 December, the ISO week-year can differ from the calendar year. For example, 31 December 2024 falls in ISO week 01 of 2025.

Does this tool use a lookup table?

No — the ISO week is computed from scratch using the standard algorithm (find the Thursday of the ISO week, then count days from 1 January).

Is the live week accurate while my tab is open?

Yes. It updates every second using your device clock. If your system clock is wrong, the displayed week will be wrong too — fix it in your OS settings.

Can I copy the week number?

Yes. Each card has a Copy button that copies the value in 'Www/yyyy' format.

Week Number Calculator

Get the ISO 8601 week number for any date — live, accurate, private.

100% private — no uploads Date & Time ToolsFree & unlimited Read tutorial

See how it works

Watch a quick simulation of this tool in action — no file needed.

How to use Week Number Calculator

Follow these steps — everything happens instantly in your browser.

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Step 1. The top card shows the current ISO week number, updating live every second.
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Step 2. Pick any date with the date picker — the bottom card shows that date's week number.
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Step 3. Each card also shows the week-year (which may differ from the calendar year near Jan 1 or Dec 31).
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Step 4. Use “Today” to snap the picker back to the current date.